1. Field of the Invention
The present invention relates generally to developing devices used in image formation apparatus, such as copiers, printers and the like, and particularly to those using a one-component developer.
2. Description of the Background Art
FIG. 16 shows a conventional electrophotography image formation apparatus with a developing device applied to a one-component development system using a one-component developer only of toner. As shown in FIG. 16, opposite to a photoreceptor 1 corresponding an image carrier, there is arranged a developing device 4 which visualizes an electrostatic latent image formed on photoreceptor 1. In general, developing device 4 supports developing roller 41 rotatably opposite in particular to an opening of a development tank 40 housing insulative toner 10. Developing roller 41 is partially exposed at the opening of development tank 40 and thus arranged for example in contact with photoreceptor 1. This contact area serves as a developing area.
One-component toner 10 is fed to a feed roller 42 and adsorbed on a surface of developing roller 41 and to control the amount of toner adsorbed there is provided a control member 43 pressed into contact with the surface of developing roller 41. Toner 10 passes under control member 43 and is thus controlled to have a predetermined amount and thus transported to the developing area borne on a surface of developing roller 41 opposite to photoreceptor 1. In accordance with the electrostatic latent image on the photoreceptor 1 surface the toner is selectively adsorbed and thus developed.
Toner 10 which has not been used in the development is transported to development tank 40 and thus recovered. To remove recovered toner 10 from the developing roller 41 surface, feed roller 42 is arranged, pressed against developing roller 41 to scrape toner 10 off the developing roller 41 surface. Then, feed roller 42 receives new toner 10 and feeds it onto the developing roller 41 surface.
Furthermore, to develop the toner satisfactorily, developing roller 41 normally receives a developing bias voltage which is set to have a voltage value allowing toner 10 to adhere to an electrostatic latent image borne on photoreceptor 1 while preventing toner 10 from adhering to an background of the image borne on photoreceptor 1.
Furthermore, to provide toner 10 adsorbed on developing roller 41 with a predetermined amount of electric charge of a predetermined polarity, the aforementioned control member 43 receives a control voltage to charge toner 10 to have the predetermined polarity. As such, toner 10 past control member 43 is provided by a fixed amount and after it is charged it is transported to the developing area.
Thus, the toner corresponding to a one-components developer can be adsorbed on the developing roller and thus transported to the developing area to allow the toner to adhere to an electrostatic latent image on the photoreceptor to form an image.
In the developing device employing triboelectricity as described above, however, the toner is rubbed against the control member only for a short period of time. As such, the toner is not charged to reach a sufficiently saturated range and each toner particle thus has a different level of electric charge. Furthermore, there are also provided toner uncharged because it has missed the chance to contact the control member, and toner inevitably charged to have a polarity opposite to a desired polarity when it is charged with triboelectricity (hereinafter referred to as "toner charged opposite in polarity"), which results in the developer having different levels of electric charge widely distributed. As such the toner cannot be developed reliably and in particular the toner charged opposite in polarity is developed at a portion other than an image. Thus the image is degraded in quality.
Furthermore, the level of electric charge has its average value significantly varying with the material(s) of the control member and that of the toner, the grain size of the toner, the environment in which the toner is used, and the like. To allow them to have their respective desired values, the material/materials added to the toner is/are adjusted extremely subtly.
Such disadvantages as above can be alleviated by using a corona charger to charge toner, as disclosed for example in Japanese Patent Laying-Open No. 10-148999.
FIG. 17 shows a developing device with a corona charger applied thereto. Toner 10, accommodated in a toner tank 40, is agitated by the rotation of an agitator 47 and transported to a developing roller 41 by the rotation of a feed roller 42. The transported toner 10 is rubbed against and thus adhered to developing roller 41 rotating in the same direction as feed roller 42 and it is rubbed against a control member 43 and thus charged and also formed in a uniform layer. The toner layer formed is more reliably charged by a charger 45 and thus develops an electrostatic latent image formed on photoreceptor 1 rotating in the same direction as developing roller 41. In this method the level of electric charge can have an average value relatively readily controlled and it can be also be provided in a narrow distribution.
The above developing device, however, also inevitably suffers toner charged opposite in polarity for the following reason: toner 10 is rubbed against control member 43 and feed roller 42 as well as each other. As such, when the toner is formed in a layer, it has a surface having a portion negatively charged and another portion positively charged. As such, when the charge level of one toner particle is noted, toner 10 is considered as being partially charged opposite in polarity.
When corona charger 45 charges the toner layer containing the toner charged opposite in polarity, toner 10 with each individual particle charged with a regular polarity as a whole but in effect partially charged with the opposite polarity, and the like, the electric charge generated by corona-discharging moves along a line of electric force, as shown in FIG. 18, and eventually adheres to toner 10. If toner 10 has a portion charged with the opposite polarity along a side thereof allowing the electric charge to adhere thereto (upwards in the figure) then toner 10 has the electric charge of the opposite polarity canceled and is eventually charged with the regular polarity. If toner 10 has a portion charged with the opposite polarity along a side thereof not allowing the electric charge to adhere thereto (downwards in the figure), however, even with a corona charger providing electric charge of a single polarity the electric charge of the opposite polarity is not canceled.
In other words, toner 10 opposite to corona charger 45 can have a desired level of electric charge through corona-charging and toner 10 not present in a surface of a toner particle, i.e., in the toner layer cannot effectively receive electric charge. Furthermore, toner 10 in a particle surface internal to the toner layer also cannot receive the electric charge provided through corona-charging. As such, the electric charge present therein and having the opposite polarity cannot be canceled and as a result only the toner surface facing the charger can have an electric charge as desired.
To overcome this disadvantage its phenomena were studied in detail and as a result there has been found a charging condition capable of canceling the electric charge of the opposite polarity existing in toner.